HP -28S Quick Reference page 38

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Also, there are the following buttons present in the solver menu. They
SOLVR: can be used to verify a solution:
Verification •
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The quit the interactive menu press any other menu key are activate the
CURSOR menu.
ISOL Isolate a unique variable in a formula. See ALGEBRA Menu.
QUAD Symbolically solves a quadratic equation. Example (flag 34 clear):
'X^2-5*X+6' 'X' QUAD returns '(5+s1)/2'.
s1 indicates an arbitrary sign so the two solutions are (5+1)/2=3 and
(5-1)/2=2.
This result is only returned when the "principal value" flag 34 has been
turned off. If it is set the solution for s1=1 is returned which is 3.
It is possible to use formal names:
'X^2-3*X-A*X+3*A' 'X' QUAD returns
'(-(-3-A)+s1*√((-3-A)^2-4*(3*A)))/2'. COLCT converts this to:
'.5*(3+√((-3-A)^2-12*A)*s1+A)'. It is easy to prove that this is
equivalent to the correct solution '.5*(3+A+s1*(A-3))' but the
calculator cannot perform this simplification.
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SHOW Show implicit references to a variable. See ALGEBRA Menu.
ROOT Root is a version of the solver that can be used in a progam.
Example: <<X DUP * 7 - >> 'X' 0 ROOT returns 2.645...
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Zero: A root has been found.
Sign reversal: Two adjacent points have been found where the
function changes it sign. Possibly, the function has a discontinuity at
this point.
Extremum: The solver found an extremum (local maximum or
minimum) or hit ±MAXR.
Bad Guess(es): When evaluating the function at the initial guess
points it causes an error.
Constant?: The function always returns the same value.
EXPR= Only present if the function is an expression or program.
When pressed the expression/program is evaluated using the current
values of the variables. The result should be 0.
LEFT= Only present if the function is an equation. When pressed it
evaluates the left side of the equation.
RT= Only present if the function is an equation. When pressed it
evaluates the right side of the equation. Left and right side should be
equal.
If A has an associated value then the result is evaluated using this
value.
Variable s1 can be set to +1 and –1 to get the two results.
If the input expression is not a polynomial of degree 2 then a Taylor
series expansion of the expression is performed up to degree 2 and
then the roots of this expansion are calculated.
Stack level 3 contains the function which can be an expression, an
equation or a program, see STEQ above.
Level 2 contains the name of the variable that the solver solves for.
Level 1 contains an initial guess or multiple initial guesses in a list as
described in SOLVR above.
The return value is a single value for X.
HP-28S
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